1. Technical Field of the Invention
The present invention relates to a method for recognizing a fine work piece and a pickup apparatus employing the method, in particular to a method for recognizing a fine work piece and a pickup apparatus employing the method, which is used, in semiconductor fabrication processes, to inspect the appearance of pellets diced in a dicing process, to determine whether the pellet is good or bad, to pick up a good pellet, and to transport it to a mounting process.
2. Description of the Related Art
The pellet mounting process, part of semiconductor device fabrication processes, includes a step for inspecting, by image recognition, the appearance of a plurality of semiconductor pellets (hereinafter referred to as pellets) that has been obtained by dividing a semiconductor wafer (hereinafter referred to as the wafer) in a grid pattern in the previous dicing process, a step for picking up only good pellets after inspection, a step for transporting the pellets to a predetermined position, and a step for mounting the pellets on a lead frame. Conventionally, methods and apparatuses employed in these steps include, for example, a technique disclosed in Japanese Patent Laid-Open Publication No.Hei 09-017,841.
FIGS. 1 and 2 show the conventional image recognition method and pickup apparatus disclosed in the above publication. A wafer 2 is divided into a grid pattern on an adhesive sheet 3, resulting in a plurality of pellets 1 arranged regularly. Thereafter, the adhesive sheet is stretched in a radial direction from the center thereof towards the circumference thereof and then attached to a wafer ring with the adhesive sheet being stretched. As the result, the pellets 1 are arranged regularly, kept at their relative positions as they were at the time of division of the wafer and spaced a little away from one another. The regularly arranged pellets 1 and the adhesive sheet 3 are loaded on an X-Y movable table 12 together with the wafer ring 4. At the time of loading the pellets 1, the center of the wafer ring 4 is so positioned as to sit on a fixed point on the X-Y coordinates of the X-Y movable table 12.
Subsequently, a drive control portion 11 drives the X-Y movable table 12 by the pitch based on the distance between the pellets 1, which has been set at a fixed value. This allows each of the pellets 1 of the wafer 2 to be scanned as shown by arrow 5 in FIG. 1, and one of the pellets 1, is recognized and positioned at a pickup position. Thereafter, the pellet 1 is imaged by means of a camera 8 and then the image signal obtained by the imaging is processed in an image-processing portion 9. The image-processing portion 9 digitizes the image signal, which is in turn stored as image data in an image storage portion 10, and determines whether the chip is good or bad, based on the data and recognizes the position of the chip from the image. Thus, the appearance of the pellet 1 is inspected. In cases where the pellet 1 has been determined to be a good pellet 1a without scratches and chips as the result of the appearance inspection, a collet 7 picks up a pellet 1a that has been determined to be good. Then, the collet 7 transports the good pellet 1a to a point above a lead frame 13 and then place the good pellet 1a on a predetermined mounting position of the lead frame 13. Thus, the pellet 1a is mounted on the lead frame 13.
The scan area of the wafer for image recognition is a circle 15 (shown by chain double-dashed lines) with a radius larger than that of the wafer 2. The scan path is shown by arrow 5 in FIG. 1. That is, the first column is scanned starting with the upper edge portion of the circle 15 in the direction of +X. When the scan has reached the circle 15, the scan of the first column is completed. Then, the scan position is shifted by one column in the direction of xe2x88x92Y along the circle 15 and thus the next column is scanned in the direction of xe2x88x92X, which is opposite to that of the previous column. This scanning pattern is repeated and thus the entire area inside the circle 15 is scanned. All of the pellets 1 are inspected by image recognition and good pellets 1a are all mounted. Thereafter, the wafer 2 is removed from the X-Y movable table 12 together with the wafer ring 4 and the next wafer 2 is positioned on the X-Y movable table 12 together with the wafer ring 4 to be placed thereon. Then, the same operation of picking up pellets 1 is repeated.
However, the range of scan is set to within the circle 15 that has a radius larger than the wafer 2. In the operation of picking up pellets 1, this causes the scan range to include regions, where no pellets exist, and the outer rim of the circumference of the wafer, where there exist not rectangular but chipped defective pellets (hereinafter referred to as non-shaped pellets). Thus, there is the problem of wasted time in the image recognition portion.
The reason why the range of scan is set to a circle having a radius larger than that of a wafer is typically because the center of the wafer does not coincide with that of the wafer ring. This is because, when the wafer is diced into a grid pattern on an adhesive sheet to be thus divided into a plurality of pellets and thereafter the adhesive sheet with the regularly arranged pellets adhered thereto is attached to the wafer ring with the adhesive sheet being stretched, the adhesive sheet cannot be always attached to the wafer ring with the center of the wafer ring being coincident with that of the wafer. Moreover, when the adhesive sheet is stretched outwards in a radial direction from the center of the wafer ring after dicing in order to ensure positive separation of pellets from one another, the center of the wafer would be displaced from that of the wafer ring.
As such, the center of the wafer is possibly displaced from that of the wafer ring when the wafer (divided pellets) is attached to the wafer ring and the adhesive sheet is stretched outwards radially from the center of the wafer. Accordingly, the area of scan had to be set to a circle larger than the wafer in order to ensure positive scanning of the entire portion of the wafer.
The object of the present invention is to provide a method for recognizing images of fine work pieces and a pick-up apparatus employing the method, which allows for eliminating waste, and reducing the time and cost required for the work by determining the scan area appropriately when images of fine work pieces into which a wafer is divided are to be recognized.
The method for recognizing images of fine work pieces according to the present invention is a method for recognizing fine work pieces, regularly arranged and obtained by dividing a wafer into the shape of grid, wafer by wafer. The method comprises the steps of recognizing images of the fine work pieces by scanning over the first wafer, setting an area constituted by good fine work pieces of the first wafer as a scan area of the second and later wafers based on results of image recognition of the first wafer, and recognizing images by scanning over the second and later wafers.
According to the present invention, when the images of fine work pieces in the wafer are recognized, the scan range of the second and later wafers is determined based on the result of recognizing the image of the first wafer, thereby allowing for limiting the scan range to a smaller one. Accordingly, for example, this allows for the reduction of the time required for the appearance inspection and pick-up processes for semiconductor pellets and the saving of memories required for storing images, thereby realizing said work with high efficiency and at low cost.